A liquid crystal display (LCD), which is a type of flat panels, has features of operating at low voltage with low power consumption, and displaying full range of colors with a compact size. The LCD has been increasingly used in wider applications ranging from watches, calculators, PC monitors, and laptops, to TVs, monitors for aviation, personal portable devices, and mobile phones.
Such an LCD panel includes two glass substrates, upon which a color filter and a thin film transistor (TFT) are formed, and a liquid crystal filled between the two glass substrates. The thin film transistor, which is a circuit where a semiconductor film is formed onto a glass substrate, adjusts a liquid crystal, and controls a pixel, which is a minimum unit constituting an image. A color filter is formed by coating pixels of three colors, red (R), green (G), and blue (B), on a glass substrate, and functions to generate images.
Various quality inspections are performed after such LCD panels are manufactured, among which one important inspection is a test for circuit patterns formed in each pixel, for example, a check for any defects by measuring dimensions, such as line widths of an electrode line and a signal line of a TFT, or circuit pattern widths. Measuring line widths is important, as line widths have an impact on adhesive strength and electrical resistance values, which affect performance of each pixel.
A conventional method of measuring line widths is performed as follows: a photographing apparatus, such as a camera, is moved to a measurement position of an LCD panel to photograph images of pixel circuit patterns; and the photographed image are input to a measurement control unit, which then processes the input images to measure pixel circuit patterns.
In the LCD panel, pixels of three colors, red (R), green (G), and blue (B), are combined to generate full-color images. As pixels have different wavelength bands depending on its colors, a problem occurs in that while a clear image may be obtained for pixels of one color, images for pixels of other colors may be less clear. In addition, image qualities may be different depending on measurement locations due to a minute curve in a glass substrate.
For this reason, a method of photographing images by adding automatic focus module to a camera is used, in which pixels are focused automatically for each color. However, clear images for every pixel cannot be acquired easily due to repeatability limitations of the automatic focus module. Further, as resolution of LCD panels becomes higher, and a circuit pattern of each pixel is miniaturized, line widths may not be measured accurately by only using an automatic focus module. Regarding an apparatus for measuring line widths, Korean Patent Publication No. 10-0521016 (published on Jul. 28, 2003) discloses such an apparatus.